The invention relates to a method of producing a denture which can be veneered with ceramic or plastics with a metallic microstructure by sintering technology, from a mixture of metal powders and if necessary glass or ceramic powders. A mixing liquid is added to the powder mixture to form a spreadable mass with which the denture is modelled using techniques conventional in dental ceramics on a ceramic model of the prepared tooth used as a bat and is subsequently sintered on the model.
Production of a metallic denture for prosthetic treatment in the case of dental sickness or after loss of one or several teeth, as for example inlays, crowns and bridges which can be veneered with ceramic or plastic, or crowns and bridges which are not veneered is done usually with the so-called "lost wax technique", a precise casting technique which ensures good dimensional accuracy.
The advantages of crowns and bridges produced in this manner include not only dimensional accuracy but primarily high strength and ductility which must be ensured in the case of larger bridge structures to prevent forced ruptures in the case of overload. On the other hand, the process itself is very time consuming, material-- and equipment--intensive. Because runner bars and spryes are necessary in the case of the lost wax method, a much higher amount of alloy must be used vis-a-vis the weight of the cast denture; this can lead to changes of alloy properties in case of multiple reuse and if not reused it remains as scrap. Another disadvantage of this technique is that in the case of defects in the cast item, repair is not possible, but the entire production process beginning with wax modeling must be repeated.
Another method for producing jacket crowns reinforced with metal caps is described in European application No. 0104320. A preformed, folded cap of a metal foil preferably built up from several layers of different metals is placed over the model of the prepared tooth and rotated onto the latter with a suitable tool. When annealed with a Bunsen burner, the superposed folds are sealed, resulting in a metal cap with a wall thickness of roughly 100 microns which is then veneered with a dental ceramic. Cost of labor and equipment is considerably reduced compared to the lost wax method; however, the denture produced in this manner falls far short of the strength properties of a cast denture so that bridges cannot be produced with this process. In addition, the uniform thickness of the metal foil in the case of highly prepared teeth or in the case of large teeth, especially molars, requires a very thick ceramic veneer so that the danger of ceramic failure is very great, especially in the case of posterior teeth.
A known method for producing full ceramic crowns is the jacket crown technique in which an aluminum oxide containing ceramic mass is applied to a platinum foil preformed to the shape of the prepared tooth and is sintered. The crown is modeled freely by hand so that the entire equipment necessary to produce cast crowns is not required. The properties of the ceramic mass allow exact modelling of the most complex tooth; shapes. The major disadvantage of this type of denture is the brittleness, a characteristic property of ceramic material, which leads to catastrophic failure in the case of sudden overload. Furthermore the strength is not adequate to produce thick walled crowns and larger bridges.
German OS No. 19 15 977 describes a method for producing a denture based on metal or alloy powders with a particle size from 2 to 25 microns which are made into a paste using a binder volatile below the sintering temperature. This paste is freely modelled using a spatula on a exact model of the prepared tooth; the model is used as a bat. The binder is expelled at higher temperatures and the metal particles sintered together. However, this method is subjected to the disadvantage that a high densification of the metal powder mixture cannot be achieved with the paste so that relatively great shrinkage takes place during sintering. The exact fit required for a denture cannot be achieved with this process, even when very fine, spherical powders are used, which on the other hand can only be produced with relatively low output and, therefore, only at high costs.